Theoretical max lumens for single or double cell light.

I'm seeing a lot of new lights boasting 1000-1100+ Lumens. Many of these are with an 18650 or two 123s. I'm under the assumption that to get more light you need to add more energy. So where are we getting these extra lumens with the same power source? What's the upper limit given the cells commonly used?

Many factors go into the max lumens you can get from a single cell light:
1. High-drain IMR 18650 cells can supply a lot of current. Much more so than older technology 18650s.
2. The choice of emitter matters a lot. One model of emitter might only produce 500 lumens of light, while another produces 1000 lumens. Typically, emitters with larger surface area are brighter. And due to advancing technology more recent model emitters are often brighter than older generation emitters.
3. The number of emitters. Emitters are more efficient when run at lower power. Lights with multiple emitters usually produce more lumens than a light of the same size and cell running a single emitter.
4. The driver. Many drivers limit the max current to the LEDs. Some more recent high-powered drivers have FETs that allow "direct drive" and basically remove this limit.

Results:
1. A small pocket-EDC size light like the Emisar D4 with 4 XPL-HI emitters, a FET driver, and a quality 18650 IMR cell like the Sony VTC6 can output 4300 lumens... but only briefly and while the cell the is at full power.
2. A slightly larger, but still-single cell light like the Fireflies E07 with a FET driver, a quality 21700 cell and a 7x XPL HI can output close to 10,000 lumens.